(1) Field of the Invention
The present invention relates to a new structure and manufacturing method of a waveguide, and more particularly, to a structure and manufacturing method of a waveguide that can be used for an optical integrated circuit.
(2) Description of the Related Art
Conventionally, waveguide structures used for optical integrated circuits include, (i) silica-based waveguides, as shown in Japanese Laid-Open Patent Publication (JP-A) No. 2002-14242A and Japanese Laid-Open Patent Publication (JP-A) No. 2001-133648A (hereinafter referred to respectively as Reference 1 and Reference 2), (ii) polymer-based waveguides, as shown in Japanese Laid-Open Patent Publication (JP-A) No. H10-3013A (hereinafter referred to as Reference 3), and (iii) Si-wire waveguides, as shown in Japanese Laid-Open Patent Publication (JP-A No. 2002-323633 (hereinafter referred to as Reference 4).
The above-mentioned (i) silica-based waveguides each have a core and a cladding layer made of silica glass (SiO2) formed on a substrate; they guide light by means of the core section doped with Ge or the like so as to have a difference in refractive index between the core and the cladding layer of about 0.2 to 5%. They are widely used for a planer lightwave circuit (PLC).
An arrayed waveguide grating (AWG), which is widely used for communications, is achieved using this technique.
The above-mentioned (ii) polymer-based waveguides each have a core and a cladding layer, both made of a polymer material, such as PMMA or polyimide; they guide light by means of a minute difference in refractive index between the material forming the core and the material forming the cladding layer.
On the other hand, the above-mentioned (iii) Si-wire waveguides each comprise a SiO2 substrate whereon a core made of Si and a cladding layer made of air or partially made of a silica glass material are formed.
The above-mentioned conventional waveguides each have strengths and weaknesses. Firsty, despite the above-mentioned (i) silica-based waveguides being used widely as a PLC technique, the radius of curvature needs to be about 1 mm, and this is comparatively small (the propagation loss is small for that part) compared to the difference in refractive index between the core and the cladding layer of less than or equal to 1%. Therefore, the confinement effect of light to the core by the difference in refractive index is weak, and a large bending loss occurs when the waveguide is tried to be bent at a radius of curvature of about less than or equal to 1 mm. Accordingly, when used in an optical integrated circuit, there is a restriction on the bending size of the waveguide and there was the problem that the optical circuit could not be made very small.
Also, although research development has been actively being undertaken towards practical use in recent years, the above-mentioned (ii) polymer-based waveguides cannot be bent at a small radius of curvature, like silica-based optical waveguides, because the difference in the refractive index between the core and the cladding layer is less than or equal to 1%.
Correspondingly, for the above-mentioned (iii) Si-wire waveguides, because the core is Si with a refractive index of about 3.5 and the cladding layer is air or SiO2 glass with a refractive index from 1 to 1.5, there is an extremely large refractive index difference between the core and the cladding layer. For this reason, in Si-wire waveguides, it is possible to bend the waveguide at an extremely small radius of curvature of several μm.
However, the cross-sectional area of the core is small at 0.2 μm×0.5 μm, and it was difficult to effectively insert light from the outside. For this reason, normally, coupling loss with a single mode optical fiber is greater than or equal to 10 dB. Also, because fundamentally the structure of the cross section differs horizontally and vertically, in Si-wire waveguides, there is polarization dependence, and it was not possible to insert and use partway through a waveguide not having polarized wave dependence, such as optical fiber. Furthermore, when a waveguide device, such as an AWG, is formed using these types of waveguide structures, because the equivalent refractive index of the waveguide is temperature dependent, the characteristics of these devices are also temperature dependent. Therefore, a temperature adjusting mechanism was necessary when using these devices. Furthermore, when a coupled waveguide is made using a conventional waveguide structure, the complete coupling length is at least several hundred μm. Therefore, when a directional coupler or optical switch is made using this, it essentially becomes as long as several mm. Furthermore, there is already known that the conventional silica-based waveguide is caused to function as a wavelength filter by being provided with a distributed reflector mechanism. In this situation, normally, a method of forming a refractive index distribution in order for Bragg reflection to occur by forming uneven grooves on the cladding layer or by using the change in refractive index by ultraviolet light is common practice. For this reason, it was necessary to have a special process in order to add a distributed reflector mechanism.